Organic light-emitting diodes (OLEDs) have reached a huge market as technology for small displays, e.g. in smartphones, and are entering the larger display and solid-state lighting markets as well. In parallel to these commercial successes, the OLED technology is adapted to a multitude of promising new applications, such as in optogenetics and medical therapy. However, it is still challenging to ensure good stability in applications that require high brightness (or high optical power density), in part due to the resulting resistive heating. Increased temperature can lead to a change in morphology of one or several organic layers, e.g. via crystallization of organic molecules, which then reduces electrical and optical performance and likely results in rapid device failure. Aside from an intrinsic resistive heating, heating can also be due to environmental effects during operation or fabrication and encapsulation of devices. For instance, atomic layer deposition (ALD) is a promising technique to form thin, yet highly protective encapsulation layers. However, state-of-the-art ALD processes require relatively high temperature during deposition (&lt; 80 °C).
4,7-diphenyl-1,10-phenanthroline (BPhen) has been widely used as electron transporting layer (ETL) due its high electron mobility, particularly in an organic matrix-dopant system. However, it is well known that thin films of BPhen tend to recrystallize spontaneously. Annealing accelerates crystallization even further due to the relatively low glass transition temperature (Tg) of BPhen (62 °C). A straightforward way to enhance the device thermal stability is to make use of a high Tg material, yet materials have to be carefully adopted to provide appropriate functionality in OLEDs.
In this contribution, we report the improvement of the thermal stability of OLEDs with BPhen based electron transport layers (ETLs) by cesium (Cs) doping. To verify the role of the Cs dopant in the BPhen matrix, recrystallization features of Cs-doped BPhen films with different doping concentrations were investigated using optical microscopy and atomic force microscopy. We also examined the photophysical properties of the films, i.e. photoluminescence (PL) and absorption. PL spectra exhibit monotonic red-shifts and broadening as the Cs doping concentration increases. This presumably indicates formation of metal complexes via interaction between the 1,10-phenanthroline group of BPhen molecules and the Cs ions. It was found that Cs plays a critical role, not only in inhibiting undesired recrystallization of BPhen molecules in a thin-film, but also in allowing BPhen layers to be thermally stable beyond the Tg of neat BPhen.
Next, the electrical and optical properties of blue and red OLEDs that contain BPhen layers with different Cs-doping concentrations as ETL were characterized after annealing at temperatures between 60 and 100 °C. We find that higher doping concentrations lead to a marked increase in thermal device stability (quantified by current density and luminance at a fixed voltage). Making use of this observation, we successfully encapsulated BPhen based OLEDs with thin-film oxide layers using ALD.
The results shown in this work may be transferable to other material systems and can thus provide a useful guideline to enhance the intrinsic thermal durability of organic devices and to render them compatible with processes involving thermal treatment.

In the past few decades Organic Light-Emitting Diodes (OLEDs) have matured to become a widespread display technology. Despite their commercial success in TVs, smartphones and tablet screens, several key issues still remain, especially if OLEDs are to become a viable technology for lighting. Metalorganic emitters based on Ir or Pt are widely used in OLED technology, due to their facile colour tunability, short phosphorescence lifetime and efficient intersystem crossing (ISC), which enables emission from both singlet and triplet states to be harnessed, leading to 100% internal quantum efficiency (IQE). However, these emitters are based on rare and toxic noble metals. In addition, efficient and stable blue and especially deep-blue phosphorescent emitters, which are essential for displays and lighting, are still to be demonstrated in order to replace currently used less efficient fluorescent blue OLEDs.
Recently, purely organic emitters exhibiting thermally activated delayed fluorescence (TADF) mechanism were also shown to be capable of 100% IQE. Since efficient ISC in these compounds is based on the small singlet and triplet gap rather than heavy atom effect, TADF compounds represent a new class of potentially inexpensive emitters for next generation OLED displays and lighting.
In this work, a series of four deep blue-to-green emitting TADF compounds have been synthesized and characterised. The compounds are based on the same scaffold as the dicarbazoyldicyanobenzene (2CzPN) reported by Adachi and co-workers, but with the replacement of the cyano groups by less electron-withdrawing oxadiazole moieties. The weaker acceptor strength of oxadiazole compared to cyano groups translates to more blue-shifted emission compared to 2CzPN. Additionally, higher spatial separation of HOMO and LUMO levels between acceptor and donor units, respectively, leads to a smaller singlet-triplet gap. This allows us to demonstrate efficient deep blue TADF OLEDs with CIE coordinates (0.16, 0.13), as compared to (0.16, 0.30) for 2CzPN devices.
A thorough photophysical study of model compound 2CzPN and oxadiazole acceptor based derivatives will be presented both for solution and thin films. The blue oxadiazole compounds show up 75% quantum yield in solid state. The electrochemical and photophysical properties, as well as crystal structures are compared to the theoretical quantum chemical calculations of the studied compounds. The efficient emission properties coupled to the small singlet-triplet gap in these molecules allows us to demonstrate efficient electroluminescence (up to 10 % EQE) from the vacuum deposited OLEDs.

Excessive charge carrier densities in the emission layer of organic light-emitting diodes (OLEDs) can lead to significant
quenching by triplet-polaron-annihilation [1] or field-induced quenching [2]. Thus, to increase the efficiency of OLEDs
further, a technique for the reliable determination of charge carrier densities in OLEDs is most desirable.
Time-resolved spectroscopy is a powerful tool to investigate electronic and excitonic transfer processes [3]. By
application of a voltage pulse to a phosphorescent state-of-the-art OLED we find a transient overshoot after voltage turn-off.
This has primarily been found in phosphorescent OLEDs and has typically been attributed to delayed recombination
of trapped charge carriers [4-7].
In this contribution we investigate charge carrier accumulation within the emission layer (EML) and provide a method to
quantify the density of stored electrons.

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